As shown in Fig. 3.13, reactors in the beginning of the operation cycle are required to have an excess reactivity compensating for the reactivity variation due to consumption of fissile materials and accumulation of FPs with burnup. Since some fuel rods of fresh fuel assemblies include pellets containing several weight percent of burnable poisons (neutron absorbers), the excess reactivity necessary at the beginning of the operation cycle is mitigated and the reactivity variation during reactor operation is lessened. In other words, the effect of burnable poisons on reactivity control is large at the beginning of the operation cycle because of the large amount of burnable poisons and neutron absorption. The reactivity variation with burnup becomes small through the balance between the reactivity decrease with burnup and the reactivity recovery due to the neutron absorption decrease with burning of burnable poisons.

Burnable poisons, with the functions noted above, are required to have a large neutron absorption cross section and gadolinia (Gd2O3) is used as a burnable poison material for BWRs. Several weight percent of gadolinia are mixed with uranium oxide (UO2) powder and processed into pellets which are inserted into several fuel rods. Since gadolinia is solid-soluble in uranium dioxide, it can be uniformly distributed in the pellets [3]. In the BWR fuel assembly design shown in Fig. 3.5, seven of 62 fuel rods are gadolinia-added fuel rods to control the excess reactivity.

Figure 3.15 shows the typical burnup characteristic of a fuel assembly containing the burnable poison gadolinia. The burnable poison-containing rods make the infinite multiplication factor small (suppression of excess reac­tivity) at the beginning of burnup when the concentrations of 155Gd and 157Gd are high. 155Gd and 157Gd are converted with burnup into 156Gd and 158Gd

Neutron Infinite Multiplication Factor without Addition of Gadolinia

Cycle Burnup

Variation m Excess Reactivity

4-Batch Core

• : Neutron Infinite

Multiplication Factor at Each BOC and EOC

Assembly Average Burnup [GWd/tJ

Fig. 3.15 Bumup characteristics of fuel assembly with burnable poison (Gadolinia)

respectively, which have small absorption cross sections, and their concentra­tions decrease. The suppression effect on the excess reactivity therefore, becomes small and the infinite multiplication factor is recovered almost to the value when not including the gadolinia-added fuel rods. Any remaining gadolinia at the end of the operation cycle will cause a reactivity loss as a useless neutron absorber, so the concentration of gadolinia is set to be burned out at the end of the operation cycle.

The number of gadolinia-added fuel rods can be increased for high suppres­sion of the excess reactivity at the initial burnup and the gadolinia concentration can be increased for long-term suppression of the excess reactivity. Figure 3.15 shows variation of the infinite multiplication factor with the average fuel assembly burnup for a 4-batch refueling; numbering corresponds to each BOC and EOC. In a core loaded with the fuel assemblies having the burnup characteristics shown in Fig. 3.15, the infinite multiplication factor of fresh fuel assemblies increases with reactor operation (0-1) and those of other fuel assemblies decrease (1-2, 2-3, and 3-4). Both characteristics compensate each other and therefore the variation in excess reactivity during an operating cycle becomes small as shown in the insert figure of Fig. 3.15. Such a proper usage of burnable poisons mitigates the work burden of control rod operation and coolant flow rate change, and considerably improves the controllability of reactor operation.